The present invention relates to a photothermographic material. In particular, the present invention relates to a photothermographic material that provides sufficient image density with a small amount of reducing agent as well as improved color tone and improved color tone change during storage.
In recent years, reduction of amount of waste processing solutions is strongly desired in the field of medical diagnosis and the field of photographic art from the standpoints of environmental protection and space savings. Therefore, techniques relating to photothermographic materials for medical diagnosis films and photographic art films are required which enables efficient exposure by a laser image setter or laser imager and formation of a clear black image having high resolution and sharpness. The photothermographic materials can provide users with a simple and non-polluting heat development processing system that eliminates the use of solution-type processing chemicals.
The same applies to the field of ordinary image-forming materials. However, photo-images for medical use require high quality excellent in sharpness and graininess as they need very fine images. In addition, for easy diagnosis, cold monochromatic images are preferred. At present, various types of hard copy systems using pigments and dyes, for example, ink jet printers and electrophotographic systems, are available as ordinary imaging systems. However, no satisfactory image-forming system is available for medical images.
Meanwhile, photothermographic systems utilizing a silver salt of an organic acid are described in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075 and Klostervoer, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d, Imaging Processes and Materials, Neblette, 8th ed., compiled by J. Sturge, V. Walworth and A. Shepp, Chapter 9, p. 279, (1989). In particular, the photothermographic material comprises a photosensitive layer containing a photocatalyst (e.g., silver halide) in a catalytically active amount, a heat developing agent, a reducible silver salt (e.g., silver salt of an organic acid), and optionally a toning agent for controlling color tone of silver, which are usually dispersed in a binder matrix. When the photothermographic material is heated at a high temperature (e.g., 80xc2x0 C. or higher) after light exposure, a monochromatic black silver image is produced through an oxidation-reduction reaction between the silver halide or the reducible silver salt (which functions as an oxidizing agent) and the heat developing agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image of silver halide generated upon exposure. Therefore, the monochromatic silver images are formed in exposed areas of the materials. This technique is disclosed in many references including U.S. Pat. No. 2,910,377 and Japanese Patent Publication (Kokoku, hereinafter referred to as JP-B) 43-4924 and Fuji Medical Dry Imager FM-DP L was put into the market as an image-forming system for medical diagnosis utilizing photothermographic materials.
The photothermographic material are characterized in that they do not require use of any process solution that necessitates troublesome management, and thus they are clean materials and can be used with a low running cost. Therefore, they are favorably accepted in the market mainly constituted by hospitals.
However, since the photothermographic materials are not subjected to a fixation treatment after the heat development, they suffer from a problem that the thermally reactive silver salt of an organic acid and reducing agent are left as they are in the photosensitive materials, and thus white portions are colored or image portions show color change when the materials are stored for a long period of time after the development. They also suffer from a problem that images after the development show bad gray balance and become bluish. If the amount of the reducing agent is reduced in order to ameliorate these problems, the maximum image density is unfavorably reduced.
Under such a technical situation, it has been strongly desired that there can be obtained gray color tone with good balance from a low density region to a high density region for silver images generated by heat development performed at a practical reaction temperature and within a practical reaction time, and such good gray color tone should not be changed by heat or light during storage. Further, conventional photothermographic materials also suffer from a drawback that they show strong environmental temperature and humidity dependency, and its improvement is desired.
Therefore, an object of the present invention is to provide a photothermographic material that can provide sufficient image density at a practical reaction temperature and within a practical reaction time, and shows little change with time after development. In particular, the object of the present invention is to provide a photothermographic material that provides, besides the aforementioned characteristics, silver color tone near pure black after the development and superior environmental temperature and humidity dependency.
The present invention provides a photothermographic material comprising a non-photosensitive silver salt of an organic acid, a photosensitive silver halide, a reducing agent for silver ions and a binder on one surface of a support, wherein content of the reducing agent is 0.4-3.5 mmol/m2, and the reducing agent contains at least one kind of compound represented by the following general formula (I): 
wherein R1 and R1xe2x80x2 each independently represent an alkyl group, at least one of which is a secondary or tertiary alkyl group; R2 and R2xe2x80x2 each independently represent hydrogen atom or a group that can be a substituent on benzene ring; L represents xe2x80x94Sxe2x80x94 group or a xe2x80x94CH(R3)xe2x80x94 group where R3 represents hydrogen atom or an alkyl group; and X and Xxe2x80x2 each independently represent hydrogen atom or a group that can be a substituent on benzene ring.
The present invention also provides a photothermographic material comprising a non-photosensitive silver salt of an organic acid, a photosensitive silver halide, a reducing agent for silver ions and a binder on one surface of a support, wherein the photothermographic material contains at least one kind of compound represented by the aforementioned formula (I) as the reducing agent, and at least one kind of compound represented by the following general formula (A): 
wherein Q represents an alkyl group, an aryl group or a heterocyclic group; X1 and X2 each independently represent a halogen atom; Z represents hydrogen atom or an electron withdrawing group; Y represents xe2x80x94C(xe2x95x90O)xe2x80x94, xe2x80x94SOxe2x80x94 or xe2x80x94SO2xe2x80x94; and m represents 0 or 1, which has a melting point not lower than a heat development temperature for the photothermographic material by more than 10xc2x0 C. but not higher than the heat development temperature by more than 55xc2x0 C.